1. Field of the Invention
This invention generally relates to a circuit of a flat panel display, and more particularly to a circuit for driving a flat panel display.
2. Description of Related Art
In the present 21st century information era, interface between users and electronic products, namely, display panels, play significant role in our everyday. Presently, traditional cathode ray tube display (CRT display), is being gradually replaced by a flat display because of its disadvantages of occupying larger space, heavier, high radiation, high power consumption compared to the flat panel displays. Accordingly, because the flat panel display is flatter and thinner occupying less space, lighter, consume less power, and provides high quality display, has become the main stream of the next generation display products. Currently, a FPD is dominated by Liquid Crystal Display (LCD). However, the LCD has several shortcomings, namely, narrow view angle and slow response speed or slow reaction. Accordingly, residual images occur when playing animation pictures. On the other hand, because the liquid crystal itself cannot illuminate and therefore a backlight module is required for illuminating the LCD display. And further, because the liquid crystals of LCD being colorless, color filters are required. Accordingly, the inclusion of backlight module and the color filters into the LCD display will increase the weight the thickness and the power consumption thereof.
Organic Luminescence Emitting Diode was disclosed in 1987 and was applied in a FPD so that the need of a backlight module can be eliminated and therefore a thinner and lighter FPD can be achieved. An Organic Luminescence Emitting diode Display, also known as OLED comprises a plurality of organic luminous elements set between two electrodes. When a current is applied to these organic luminous elements via the electrodes, light is emitted. Since the luminance or the brightness of the organic luminescence emitting diode is proportional to applied current, and therefore any current variation will directly affect the uniformity of OLED illumination. Because a general voltage driven pixel cannot compensate illumination non-uniformity among TFT pixels, and therefore it is commonly believed that the current driven pixel provides better illumination uniformity.
FIG. 1A is a circuit diagram illustrating a conventional circuit for driving a flat panel display. FIG. 1B is a timing diagram illustrating voltage or current the signals of FIG. 1A. As shown in FIG. 1A, a current-driven pixel generally comprises a storage capacitor 131, which is adapted data to store a voltage. The storage capacitor 131 accumulates the voltage during scanning signal Scan. That is, when the scanning signal Scan switches from a high voltage level to a low voltage level, the transistors 101 and 103 are turned on, the transistor 102 is turned off, a voltage Va is stored into the storage capacitor 131 as a data signal Data. The voltage Va is the voltage at node a in FIG. 1A, which determines the resistance of the transistor 105. When the scanning signal Scan is cut off (i.e. switching from low to high voltage level), the transistors 101 and 103 are turned off, and the transistor 102 is turned on, the transistor 105 controls the current density flowing through the OLED diode according to the stored voltage Va. In other words, the stored voltage Va in the storage capacitor 131 indirectly determines the luminance intensity of OLED diode.
In practical application, when the scanning signal Scan is cut off, the original charge stored at node a is affected due to feed-through effect in a manner that the stored voltage Va is changed as well. When the stored voltage Va is changed, the current density flowing through the transistor 105 and the organic luminescence emitting diode (OLED) is correspondingly changed. Thus grayscale distortion problem would occur since pixels are of low uniformity. Yet if the capacitance of the storage capacitor Cs is enlarged for intervening feed-through effect, the pixel-opening rate is reduced, large chip area is consumed and the response speed or the reaction speed is lowered. Accordingly, this approach is not desirable.